The catalyst of this kind is enabled, by the oxygen storage capability (OSC) of ceria even when the exhaust gas composition near the catalyst component fluctuate in some measure, to oxidize the carbon oxide (CO) or the hydrocarbons (HC) contained in the exhaust gas and to reduce the nitrogen oxides (NOx). Thus, it is known that the catalyst containing ceria is excellent in the purifying ability of the exhaust gas. Since the absorption of oxygen is an exothermic reaction, moreover, the temperature rise of the catalyst can be promoted to exhibit the catalytic activity early. Therefore, it is also known that the ceria is excellent as assistant catalyst.
In case the ceria is employed in the coating layer, another metal oxide is employed together so as to keep the stability. Specifically, the ceria is used as a compound oxide with zirconium (Zr). An example of the cerium (Ce)-zirconium (Zr) compound oxide is disclosed in Japanese Patent Laid-Open No. 10-194742.
Here, the cerium-zirconium composite oxide is used as an assistant catalyst in not only a ternary catalyst but also a storage reduction type NOx purifying catalyst. Specifically, the storage reduction type NOx purifying catalyst carries such a NOx storage material together with the cerium-zirconium composite oxide as stores the NOx as a nitrate in an oxidizing atmosphere but releases the NOx in a reducing atmosphere. If the catalyst of this kind is employed as an exhaust gas purifying catalyst for the engine, in which the generation of NOx is augmented by the so-called “lean burning run” at a high air/fuel ration for improving the fuel economy, the purification can be performed by causing the NOx in the oxidizing atmosphere to be stored as a nitrate by the NOx storage material and the NOx in the reducing atmosphere to be released and reduced.
In the storage reduction type NOx purifying catalyst thus far described, the sulfur oxide contained in the exhaust gas is further partially oxidized by the catalytic particles of Pt or the like, and is stored in the form of sulfate by the NOx storage material. This sulfate is so chemically stabler than carbonate or nitrate that it is not easily released even in a high-temperature reducing atmosphere from the NOx storage material. In other words, the storage reduction type NOx purifying catalyst of the prior art is so easily poisoned with sulfur as to raise a problem that its catalytic activity is lowered with time.